Modern electrical distribution circuits are fed from a step down transformer which takes in higher transmission voltages (e.g., 1000s of volts) used for transmission and converts the voltages to lower, more usable voltages. As a general rule, distribution networks are designed with higher amperage and voltage rated circuit protection devices closer to the transformer and lower amperage and voltage rated protection devices further away from the transformer. An example of a simple power distribution network 100 is illustrated in FIG. 1. In the example, a step down transformer 102 is coupled to a “downstream” 2000 amp circuit breaker 104 which protects three downstream 800 amp circuit breakers 106, 108, 110 in parallel on three branches. One of the 800 amp circuit breakers 106 protects three downstream 250 amp circuit breakers 112, 114, and 116 in parallel on three sub-branches. Finally, one of the 250 amp circuit breakers 116 protects three downstream 160 amp circuit breakers 118, 120, and 122 in parallel on three sub-sub-branches. With each downstream step, the amount of amperage protection required is reduced. When an electrical fault event occurs, it is desirable to maintain operation of as much service as possible to the remaining parts of the network 100. When the network 100 is able to isolate the fault event and maintain service to the rest of the network 100, the application is known as “selective.” When power is lost to an unaffected part of the network 100 (i.e., the fault event only occurs on a sub-branch but causes all the upstream breakers to trip) the application is “non-selective.”
FIGS. 2A and 2B more clearly illustrate the difference between a non-selective and a selective application, respectively. FIG. 2A depicts a non-selective network 200A in which a fault event 202 has caused circuit breakers 122, 116, and 106 to all trip which results in an outage 204 that leaves most of the network 200A without electrical service. In contrast, FIG. 2B depicts a selective network 200B in which a fault event 202 has caused only circuit breaker 122′ to trip which results in an outage 204′ that only leaves a relatively small portion of the network 200B without electrical service.
In order to be selective, the circuit protection devices (e.g., circuit breakers) must identify where the electrical fault has occurred and act accordingly as fast as possible. This means that an upstream breaker must be able to distinguish between a fault that occurs nearby, and one that occurs downstream of another breaker. A selective power distribution system means lower downtime costs for the electrical service customer and a more stable distribution network even when problems occur. In the past, this has been achieved using tripping characteristic curves.
An example of a tripping characteristic curve 300 used for selective coordination of circuit protection devices is illustrated FIG. 3. A first curve 302 represents the behavior of a downstream circuit breaker and a second curve 304 represents the behavior of a circuit breaker located immediately upstream from the downstream circuit breaker. Using tripping characteristic curves 300 to implement selectivity has many limitations which are mainly governed by the physical attributes of the contact system within the circuit breakers.
Some manufacturers have attempted to add extra devices inside the circuit breakers to increase the ability of the breaker to distinguish where an electrical fault has occurred in the dynamic breaker behavior region 306 of the tripping characteristic curve. These extra devices are typically designed as integrated components of the breaker and for the specific physical attributes of the contact system within a particular circuit breaker. These devices are not applicable or re-useable for different circuit breakers. For example, a prior art circuit breaker that includes an integrally formed, non-removable, and non-modular selectivity device is the Model Tmax T6 circuit breaker manufactured by ABB Asea Brown Boveri Ltd of Zurich, Switzerland. This example breaker includes a custom designed, integral selectivity device developed based on tripping characteristic curves. Thus, what is needed are methods and apparatus for an improved magnetic armature selective tripping device that is modular and can be easily configured for use in different circuit breakers.